In wireless communication systems, accurate channel coefficients are needed for many purposes. Channel coefficients are e.g. used for the coherent combining weight of data received with different delays used for user and control data decoding in rake receivers. Channel coefficients are also used for Signal-to-Interference (SIR) estimation for inner loop power control as well as for load estimation. The demands to have very accurate channel estimates increase when the requests for higher data transmission rates increase.
A typical UpLink (UL) Wideband Code Division Multiple Access (WCDMA) receiver of today estimates the multipath channel properties. Delays and coefficients of each user are estimated by using known pilot bits and other information bits transmitted on a Dedicated Physical Control CHannel (DPCCH). The DPCCH channel is spread with a channelization and scrambling code. A spreading factor of 256 is commonly used. It is common to use a searcher to first identify the multipath delays that contain significant power and then despread the received signal at those delays.
The despread signals are in the following referred to as rake fingers and may typically originate from more than one antenna. A model for the despread DPCCH symbol is created. The parameters of the model are tracked by a tracker for each time slot. Changes between one slot and a following slot, e.g. according to different fading mechanisms, will thereby cause the tracker to adapt the parameters accordingly. For stationary or very slowly moving user equipments, the fading is typically small and the tracker will have a better possibility to provide very accurate estimations of the channel. In cases where tracking with an integrated random walk approach the step size can typically be decreased.
However, in WCDMA, transmit power control (TCP) is applied. This means that at each time slot, a TCP command is transmitted to a user equipment commanding the user equipment to increase or decrease the UL transmit power with a certain amount. Presently, in WCDMA, the UL transmit power is increased or decreased by 1 dB in each time slot. This change in transmit power will add to the fading of the channel, when the channel is going to be estimated.
One problem with prior art trackers is that they do not take into account the TPC commands that increase/decrease the power every slot. The drawback of this is that the tracker sees the TPC commands as part of the integrated random walk model and increases the step size. As a consequence, the tracker uses less filtering than what is possible, resulting in poorer channel estimation quality than necessary. This is especially important for stationary, high rate users where accurate channel estimation is needed and long channel averaging periods are beneficial.